Structure and hydrology of the Ogiri field, West Kirishima geothermal area, Kyushu, Japan

The geothermal system in the West Kirishima area is controlled by a system of faults and fractures oriented along two main directions, northwest to southeast and east–northeast to west–southwest. The Ginyu fault extends through the Ogiri field in the Ginyu area, which is one of the east–northeast to west–southwest striking faults in this area. This fault is the reservoir target for developing the geothermal resources in the Ogiri field. The Ginyu fault is a near planar fracture with a uniform temperature of 232°C and has near-neutral pH, chloride fluids. Based on the results of a detailed analysis of the Ginyu fault, all production wells drilled in the Ogiri field intersected the Ginyu fault reservoir successfully, securing steam production for a 30 MW_e power plant. A typical fracture-type geothermal model for the Ogiri field was developed on the basis of the geology, electric and geophysical logs, fluid chemistry, and well test data.     

Chemical and physical reservoir parameters at initial conditions in Berlingeothermal field,El Salvador: a first assessment

A study has been madeto obtain the main chemical and physical reservoir conditions of the Berlin field (El Salvador)before the commencement of large-scale exploitation of the geothermal resource The upflowzone and the main flow path within the geothermal system have been determined from the arealdistribution of chemical parameters such as Cl concentrations ratios such as Na/KK/Mg,K/Ca,and temperatures computed from silica concentrations and cation ratios Gas compositions havebeen used to calculate reservoir parameters such as temperature steam fraction and P_CO2 The computer code WATCH (new edition 1994) hasbeen used to evaluate the temperature of equilibration between the aqueous species and selectedalteration minerals in the reservoir The fluid in Berlin flows to the exploited reservoir from thesouth entering it in the vicinity of well TR-5 Along its flow-path (south–north direction) thefluid is cooled by boiling and conductive cooling The chloride-enthalpy diagram indicates theexistence of a parent water with a chemical composition similar to well TR-5 that boils and theresidual brine produces the fluid of well TR-3 which is very concentrated in salts The fluid ofTR-5 is probably produced from this parent water generating the fluids of wells TR-2 and TR-9by boiling and the fluids of wells TR-1 and TR-4 by conductive cooling The computed values forthe deep steam fraction clearly indicate that this is a liquid-dominated system with computedtemperature values decreasing from 310°C (upflow zone) to about 230°C from south to north© 1999 Published by Elsevier Science Ltd on behalf of CNR All rights reserved     

Geothermal resources of Hachijojima

Hachijojima is a gourd-shaped volcanic island in the Pacific Ocean. Nishiyama and Higashiyama volcanoes consist of basalt lava and associated pyroclastic rocks. A promising geothermal resource was found in south Higashiyama, associated with an uplift of Tertiary rocks consisting of mainly andesite lava and related pyroclastic rocks, overlain by Quaternary volcanic rocks. Steep high-temperature (over 250°C) and high-pressure gradients occur in the deeper portion of the system near the Tertiary–Quaternary contact, indicating the presence of a cap rock. The cap rock formed by deposition of hydrothermal minerals. Geothermal fluid ascends from the deeper portions to shallow depths along vertical fractures through the cap rock. These vertical fractures form the geothermal reservoir in the Tertiary formation. Three wells were drilled into these vertical fractures, and approximately 30 t/h of superheated steam was obtained from each well during flow tests. The geothermal fluid is mainly a mixture of seawater and meteoric water in an approximate ratio of 1 to 2, based on chemical analyses, with a portion of volcanic gas included. At present a 3.3 MW_e, geothermal power plant is being constructed here.     

Monitoring production using surface deformation: the Hijiori test site and the Okuaizu geothermal field,Japan

Production in geothermal reservoirs often leads to observable surface displacement. As shown in this paper, there is a direct relationship between such displacement and reservoir dynamics. This relationship is exploited in order to image fluid flow at two geothermal field sites. At the first locality, the Hijiori Hot Dry Rock (HDR) test site, 17 tilt meters record deformation associated with a 2.2 km deep injection experiment. Images of fluid migration along a ring fracture system of the collapsed Hijiori caldera are obtained. At the Okuaizu geothermal field, leveling and tilt meter data provide constraints on long- and short-term fluid movement within the reservoir. A set of 119 leveling data suggest that the north-to-northeast trending Takiyagawa fault acts as a barrier to flow. The northwesterly oriented Chinoikezawa and Sarukurazawa faults appear to channel fluid from the southeast. The tilt data from Okuaizu indicate that a fault paralleling the Takiyagawa fault zone acts as a conduit to transient flow, on a time scale of several weeks. The volume strain in a region adjacent to the injection wells reaches a maximum and then decreases with time. The transient propagation of fluid along the fault may be due to pressure build-up, resulting from the re-initiation of injection.     

Temporal and spatial distribution of local seismicity in the Chipilapa-Ahuachapán geothermal area, El Salvador

Microseismic monitoring of the Chipilapa-Ahuachapán area was carried out during August-November 1988 and October 1991–April 1992. The objective was to use the study of microearthquakes as an exploration tool to invvestigate the geothermal potential of the Chipilapa area and to evaluate the main characteristics of the seismic activity, prior to and during the exploitation tests. Since 1989, seven wells have been drilled in the area, two of which have encountered three geothermal aquifers that could be exploited for electricity generation by means of binary-cycle technology. The 1988 survey detected important, shallow and low magnitude seismic activity, located mainly south and southwest of the explored area. This activity is possibly related to the recharge zone of the Chipilapa-Ahuachapán geothermal system, located further south, beneath the Pleistocene Pacific Volcanic Chain. The 1991–1992 survey confirmed the existence of seismicity beneath the southern volcanic axis, but other important clusters of activity were recorded northward, related to the deeper structures of the Central Graben, and southwest of the Ahuachapán geothermal field, close to the 1990 hydrothermal eruption of Agua Shuca. Shallow microseismic activity also appeared along the faults limiting the Chipilapa geothermal field to the east. Although it is probable that this seismicity is due to fluid circulation in fractures, no geothermal reservoirs were intercepted by wells CHA and CH8. Moreover, no significant induced seismicity was recorded during production and injection tests.     

Introduction to geothermal exploration on ascension island, South Atlantic Ocean

The exploration for a geothermal resource on Ascension Island utilized a strategy that initially employed geologic mapping. On the basis of this, subsurface faults were mapped using an aeromagnetic survey. The faults were then explored using electrical resistivity surveys to define areas of potential hydrothermal fluid up-welling. The results of all of these techniques were used to site temperature gradient holes. A deep geothermal exploration well was then drilled in the area with highest heat flow adjacent to a rhyolite-to-trachyte volcanic complex.     

Characteristics and management of the Sumikawa geothermal reservoir, northeastern Japan

The subsurface temperature gradually increases southward in the Sumikawa geothermal field and decreases sharply toward the north. The geothermal reservoir contains a two-phase zone between the cap rock and hot water zone. The target for production was designated in the deep zone, in the high temperature southern area. The production and reinjection areas have been separated to recover thermal energy efficiently during the recycling of reinjection fluid; the wells have been spaced as far apart as possible to reduce well interference. To improve productivity and injectivity, cold-water well stimulation was applied, and this experiment reduced the number of wells required for 50 MW_e power generation.     

Reservoir engineering assessment of Dubti geothermal field,Northern Tendaho Rift,Ethiopia

Following on from surface exploration surveys performed during the 1970s and early 1980s, exploration drilling was carried out in the Tendaho Rift, in Central Afar (Ethiopia), from October 1993 to June 1995. Three deep and one shallow well were drilled in the central part of the Northern Tendaho Rift to verify the existence of a geothermal reservoir and its possible utilisation for electric power generation. The project was jointly financed by the Ethiopian Ministry of Mines and Energy and the Italian Ministry for Foreign Affairs. Project activities were performed by the Ethiopian Institute of Geological Surveys and Aquater SpA. The main reservoir engineering data discussed in this paper were collected during drilling and testing of the above four wells, three of which are located inside the Dubti Cotton Plantation, in which a promising hydrothermal area was identified by surface exploration surveys. Drilling confirmed the existence of a liquid-dominated shallow reservoir inside the Dubti Plantation, characterised by a boiling-point-for-depth temperature distribution down to about 500 m depth. The main permeable zones in the Sedimentary Sequence, which is made up of lacustrine deposits, are located in correspondence to basalt lava flow interlayerings, or at the contact between volcanic and sedimentary rocks. At depth, the basaltic lava flows that characterise the Afar Stratoid Series seem to have low permeability, with the exception of fractured zones associated with sub-vertical faults. Two different upflows of geothermal fluids have been inferred: one flow connected to the Dubti fault feeds the shallow reservoir crossed by wells TD-2 and TD-4, where a maximum temperature of 245 °C was recorded; the second flow seems to be connected with a fault located east of well TD-1, where the maximum recorded temperature was 270 °C. A schematic conceptual model of the Dubti hydrothermal area, as derived from reservoir engineering studies integrated with geological, geophysical and geochemical data, has been tested by numerical simulation, using the TOUGH2/EWASG code. Preliminary simulations, using a simple 3-D numerical model of the Dubti fault area, showed that measured temperature and pressure distribution, as well as evaluated non-condensable gas pressure at reservoir conditions, are compatible with the rise of geothermal fluid, at about 290 °C, along the sub-vertical Dubti fault from beneath the surface manifestations DB1, DB2 and DB3 located at the south-eastern end of the fault. According to the proven shallow field potential, development of this field could meet the predicted electricity requirements of Central Afar until the year 2015.     

Distribution of hydrothermal mineral zones in the Cerro Prieto geothermal field of Baja California, Mexico

Our ongoing studies of water-rock interaction at Cerro Prieto have now been extended to include samples from 40 wells. We have confirmed the regular sequence of progressive hydrothermal alteration zones previously described, and have mapped these alteration zones across the geothermal field. Our earlier work showed the relationships between hydrothermal mineralogy, temperature and permeability, in that alteration occurs at lower temperature in sandstone than in the less permeable shales. The effects of chemical parameters such as silica activity and differences in CO₂ fugacity have also become apparent when mineral assemblages are compared in sandstones from different wells at the same temperature. A rather complete picture of the shape of the reservoir and the nature of its boundaries is developing, and we have begun to identify patterns in the observed hydrothermal mineral zonations which are characteristic of different temperature gradients. We infer such different gradients to be indicative of different parts of the hydrothermal flow regime. In certain wells mineral zones are closely spaced, indicating steep thermal gradients, while in others they are much farther apart. We believe that patterns characteristic of recharge, discharge and upwelling zones as well as areas of primarily horizontal flow can be recognized.The geothermal circulation system at Cerro Prieto appears to be rather young and shows no indication of retrograde reactions due to cooling. The pattern of fluid flow does not seem to be significantly affected by faults, stratigraphic horizons or by the presence of a cap-rock. The mineral zones define a thermal dome which is apparently fed from the east and spreads westward.     

Drilling and evaluation of Ascension #1, a geothermal exploration well on Ascension Island, South Atlantic Ocean

A deep (3126 m) geothermal exploration well (Ascension #1) was drilled on Ascension Island in the South Atlantic Ocean as the culmination of an exploration program that began in 1982. Ascension #1 encountered several geothermal fluid entries below a depth of 2400 m, and had a bottomhole temperature approaching 250°C. However, the fluid flow rate was limited. While attempting to improve production by drilling a second leg, a mechanical failure resulted in loss of the well. An analysis of the geologic controls on fluid production suggests that fracture permeability is oriented to the northeast and often associated with felsic dikes. The system may be sealed by a thick sequence of hyaloclastites that are mechanically unable to sustain open fractures. The reservoir intersected by Ascension #1 apparently lacks the permeability required for commercial fluid production.     

Overview of the Wayang Windu geothermal field,West Java,Indonesia

The Wayang Windu geothermal field, West Java, Indonesia, is interpreted to be transitional between vapour-dominated and liquid-dominated conditions with four coalesced fluid upwelling centres that generally become younger and more liquid-dominated towards the south. Two of these centres are associated with the large Gunung Malabar andesite stratovolcano and the other two with the smaller aligned Gunung Wayang and Gunung Windu andesitic volcanoes to the south. The overall potential resource area is of the order of 40 km². Deep wells encounter a deep liquid reservoir whose top, which ranges from 0 to 400 m above sea level (m asl) becomes progressively deeper toward the south. As pressure versus elevation conditions are the same throughout the deep liquid reservoir it is likely to be contiguous. This liquid-dominated reservoir is overlain by three separate vapour-dominated reservoirs. The northernmost is the largest as it is coalesced over two separate fluid upwelling centres. Its low gas content, size, prolonged productivity and isobaric for elevation nature, preclude it from being a parasitic steam zone. Mineralogical relationships demonstrate that this vapour zone was originally liquid-dominated with a deep water level as high as 1700 m asl. Subsequent boil off may reflect low recharge rates due to hydrological isolation at depth. To the south, the vapour-dominated reservoirs decrease in thickness and are characterized by progressively higher pressures, temperatures and gas contents. These changes suggest that the southernmost vapour-dominated zone is the youngest and that these zones become increasing older to the north.     

Heat flow in Albania

As part of a joint geothermal project between Albania and the Czech Republic, a field expedition was organized to Albania in summer 1993 to measure temperature profiles in selected boreholes and to collect rock samples to determine their thermal conductivity. Fourteen localities were visited and nine detailed temperature-versus-depth profiles were obtained. These results were completed with numerous industrial temperature records from other deep holes. The regional patterns of temperatures at 100 m depth and of characteristic near-surface temperature gradients were constructed, and the effect of the topography on the subsurface temperature field was assessed to calculate a total of 49 heat flow density data. A generally low geothermal gradient exists in all of the country, ranging from 7–11 mK/m in the synclinal belt, 11–13 mK/m in southernmost Albania, to a maximum of 18–20 mK/m in the central part of the Pre-Adriatic Depression. These values correspond to a low heat flow zone of 30 to 45–50 mW/m² extending from the north and bordering the Adriatic coast. Heat flow generafly increases from west to east, but its distribution in the Inner Albanides is not clear.     

Changes in thermodynamic conditions of the ahuachapán reservoir due to production and injection

Since large-scale exploitation of the Ahuachapán reservoir began in 1975 large changes in the reservoir thermodynamic conditions have occurred. Drawdown of up to 15 bars and significant temperature changes have been observed in the wellfield. Temperatures have declined due to boiling in the reservoir in response to the pressure drawdown; localized and minor cooling due to reinjection of spent geothermal fluids have also been observed. There are indications of cold fluid influx deep into the reservoir from the west and north. Reservoir temperatures show that a significant amount of hot fluid recharge comes to the wellfield from the southeast, and temperatures also indicate that the recharge rate has increased with time as pressure declines in the reservoir. Chemical analyses of the produced fluids show that most wells are fed by a mixture of geothermal fluids and cooler, less-saline waters. The cold water inflow has increased due to exploitation, as demonstrated by decreased salinity of the produced fluids.     

Intersecting faults and sandstone stratigraphy at the cerro prieto geothermal field

The NW - SE trending Cerro Prieto fault zone is part of a major regional lineament that extends into Sonora, Mexico, and has characteristics of both a wrench fault and an oceanic transform fault. The zone includes a number of separate identified faults and apparently penetrates deep into the basement and crustal rocks in the area. The zone serves as a conduit for both large and rapid heat flow. Near well M-103, where the Michoacán fault obliquely intersects a shorter NE - SW trending fault (i.e., the Pátzcuaro fault), large circulation losses during drilling indicate greater permeability and hence increased natural convective fluid flow. Temperature contour maps for the southern portion of the field suggest that a shear fault zone also exists in the vicinity of wells M-48, M-91 and M-101. This shear zone aids in rapidly distributing geothermal fluid away from the Cerro Prieto fault zone, thus enhancing recharge to the western part of the reservoir.We have studied the distribution of lithologies and temperature within the field by comparing data from well cuttings, cores, well logs and geochemical analyses. Across the earliest developed portion of the field, in particular along a 1.25 km NE - SW section from well M-9 to M-10, interesting correlations emerge that indicate a relationship among lithology, microfracturing and temperature distribution. In the upper portion of the reservoir of this section, between 1200 and 1400 m, the percentage of sandstones ranges from 20 to 55. Well logs, calcite isotope maxima, and the Na - K - Ca geothermometer indicate temperatures of 225–275°C. The isothermal high in this vicinity corresponds to the lowest total percentage of sandstones. Scanning electron microphotographs of well cores and cuttings from sandstone and shale units reveal open microfractures, mineral dissolution and mineral precipitation along microfractures and in pores between sand grains. Our working hypothesis is that these sandy shale and siltstone facies are most amenable to increased microfracturing and, in turn, such microfracturing allows for higher temperature fluid to rise to shallower depths in this part of the reservoir.Our ongoing research is aimed at achieving a coherent geological model that provides a basis for estimating reservoir capacity, and that illustrates our understanding of fluid flow along major faults, laterally through fault shear zones, and within predominantly silty and shaley deltaic clastics that have been microfractured.     

Hydrogeochemical exploration of geothermal prospects in the Tecuamburro Volcano region, Guatemala

Chemical and isotopic analyses of thermal and nonthermal waters and of gases from springs and fumaroles are used to evaluate the geothermal potential of the Tecuamburro Volcano region, Guatemala. Chemically distinct geothermal surface manifestations generally occur in separate hydrogeologic areas within this 400 km² region: low-pressure fumaroles with temperatures near local boiling occur at 1470 m elevation in a sulfur mine near the summit of Tecuamburro Volcano; non-boiling acid-sulfate hot springs and mud pots are restricted to the Laguna Ixpaco area, about 5 km NNW of the sulfur mine and 350–400 m lower in elevation; steam-heated and thermal-meteoric waters are found on the flanks of Tecuamburro Volcano and several kilometers to the north in the andesitic highland, where the Infernitos fumarole (97°C at 1180 m) is the primary feature; neutral-chloride hot springs discharge along Rio Los Esclavos, principally near Colmenares at 490 m elevation, about 8–10 km SE of Infernitos. Maximum geothermometer temperatures calculated from Colmenares neutral-chloride spring compositions are 180°C, whereas maximum subsurface temperatures based on Laguna Ixpaco gas compositions are 310°C. An exploration core hole drilled to a depth of 808 m about 0.3 km south of Laguna Ixpaco had a bottom-hole temperature of 238°C but did not produce sufficient fluids to confirm or chemically characterize a geothermal reservoir. Hydrogeochemical data combined with regional geologic interpretations indicate that there are probably two hydrothermal-convection systems, which are separated by a major NW-trending structural boundary, the Ixpaco fault. One system with reservoir temperatures near 300°C lies beneath Tecuamburro Volcano and consists of a large vapor zone that feeds steam to the Laguna Ixpaco area, with underlying hot water that flows laterally to feed a small group of warm, chloriderich springs SE of Tecuamburro Volcano. The other system is located beneath the Infernitos area in the andesitic highland and consists of a lower-temperature (150–190°C) reservoir with a large natural discharge that feeds the Colmenares hot springs.     

Use of slim holes with liquid feedzones for geothermal reservoir assessment

Production and injection data from slim holes and large-diameter wells at four geothermal fields (Oguni, Japan; Sumikawa, Japan; Takigami, Japan; Steamboat Hills, U.S.A.) were analyzed in order to establish relationships (1) between injectivity and productivity indices, (2) between productivity/injectivity index and borehole diameter, and (3) between discharge capacity of slim holes and large-diameter wells. The productivity and injectivity indices for boreholes with liquid feedzones are more or less equal. Except for the Oguni boreholes, the productivity and injectivity indices display no correlation with borehole diameter. Thus, the productivity index (or, more importantly, the injectivity index in the absence of discharge data) from a slim hole with a liquid feed can be used to provide a first estimate of the probable discharge capacity of a large-diameter geothermal production well. The large-diameter wells at the Oguni, Sumikawa and Steamboat Hills geothermal fields have a more or less uniform inside diameter, and the discharge capacity of these wells (with liquid feedzones) can be predicted using Pritchett's “scaled maximum discharge rate” in conjunction with discharge data from slim holes. Because of the non-uniform internal diameter for large-diameter Takigami wells, it is not possible to use a simple scaling rule to relate the discharge capacities of slim holes and large-diameter wells at Takigami; therefore, a numerical simulator was used to model the available discharge data from Takigami boreholes. The results of numerical modeling indicate that the flow rate of large-diameter Takigami production wells with liquid feedzones can also be predicted using discharge and injection data from slim holes.     

Effect of tectonics and earthquakes on geothermal activity near plate boundaries: A case study from South Iceland

We studied fracture-controlled geothermal fields in the Hreppar Rift-Jump Block (HRJB), a micro-plate bounded by two NNE rifts and the E–W transform zone of the South Iceland Seismic Zone (SISZ). Distinguishing whether the extensional rift swarm or the transform zone shear fractures host the geothermal activity is challenging. GPS mapping of 208 springs and tectonic analysis indicate that six Riedel shear fracture sets of an older transform zone in the HRJB are permeable. Northerly dextral strike-slip faults are the principal permeable faults, although the highest discharge and temperature are found at their intersections with other fracture sets. Two northerly faults from the HRJB connect to the source faults of the major 1784 and 1896 earthquakes within the active SISZ. The 1784 earthquake caused pressure changes as far north as the studied springs, indicating that earthquakes keep faults permeable over hundreds of years.     

Heat flow and deep temperatures in the Chaves geothermal system,Northern Portugal

The geological, geoelectrical, geochemical and temperature data related to the Chaves geothermal system have been integrated to obtain a better understanding of the Chaves basin. Geoelectrical surveys carried out in the basin reveal a low-resistivity zone (10 ohm m), associated with a shallow geothermal reservoir, in the central part of the graben, bounded by higher-resistivity rocks. The top of this zone varies between 400 and 200 m and its maximum thickness (1600 m) is located at the centre of the basin. Thermal models for the Chaves basin and for the region are presented using the structure obtained by geoelectrical methods and a mean heat flow value of 95 mW m^-2 derived from borehole measurements. The heat transfer takes place mainly by conduction, except near the faults, where convective flow is important. The medium is considered dishomogeneous and there is a great thermal conductivity contrast between the sediments in the basin and the surrounding rocks. The results obtained for the Chaves basin show that the mean temperature value in the shallow geothermal reservoir is 62°C. The maximum temperature value predicted to the bottom of this reservoir is 95°C. A regional forced convective-circulation model is suggested based on geomorphological, geochemical, isotopic data and to rmal models.     

Simulation of heat extraction from crystalline rocks: The influence of coupled processes on differential reservoir cooling

Processes operating during the extraction of heat from fractured rocks influence dynamically their fluid flow and heat transport characteristics. The incorporation of pressure- and temperature-dependent rock parameters, coupled with geomechanical deformation, is particularly important for predictive modelling of geothermal reservoirs hosted in crystalline rock masses. Changes in flow and transport parameters of fractures caused by variations in local effective stress are computed using an experimentally validated geomechanical model [McDermott, C.I., Kolditz, O., 2006. Geomechanical model for fracture deformation under hydraulic, mechanical and thermal loads. Hydrogeol. J. 14, 487–498]. Local effective stress changes are linked to alterations in reservoir fluid pressures, and to in situ stress conditions, including the build-up of thermal stresses resulting from the cooling of the rock mass. These processes are simulated using a finite-element model in order to study the behaviour of the Spa Urach (southwestern Germany) potential geothermal reservoir. The model couples mechanical deformation and alteration of fracture parameters with pressure-, temperature- and salinity-dependent fluid parameter functions. The effects of potential reservoir damage on reservoir productivity are investigated to help identify optimal heat recovery schemes for the long-term economical exploitation of geothermal systems. Simulation results indicate that preferential fluid flow paths and shortcuts may develop, depending on the mechanical and thermal stress releases that occur during intense exploitation of these systems.     

Controls on the Karaha–Telaga Bodas geothermal reservoir,Indonesia

Karaha–Telaga Bodas is a partially vapor-dominated, fracture-controlled geothermal system located adjacent to Galunggung Volcano in western Java, Indonesia. The geothermal system consists of: (1) a caprock, ranging from several hundred to 1600 m in thickness, and characterized by a steep, conductive temperature gradient and low permeability; (2) an underlying vapor-dominated zone that extends below sea level; and (3) a deep liquid-dominated zone with measured temperatures up to 353 °C. Heat is provided by a tabular granodiorite stock encountered at about 3 km depth. A structural analysis of the geothermal system shows that the effective base of the reservoir is controlled either by the boundary between brittle and ductile deformational regimes or by the closure and collapse of fractures within volcanic rocks located above the brittle/ductile transition. The base of the caprock is determined by the distribution of initially low-permeability lithologies above the reservoir; the extent of pervasive clay alteration that has significantly reduced primary rock permeabilities; the distribution of secondary minerals deposited by descending waters; and, locally, by a downward change from a strike-slip to an extensional stress regime. Fluid-producing zones are controlled by both matrix and fracture permeabilities. High matrix permeabilities are associated with lacustrine, pyroclastic, and epiclastic deposits. Productive fractures are those showing the greatest tendency to slip and dilate under the present-day stress conditions. Although the reservoir appears to be in pressure communication across its length, fluid, and gas chemistries vary laterally, suggesting the presence of isolated convection cells.